A crucial period in the history of the → Universe,
when the bulk of stars in massive galaxies were
likely formed. Observations of young stars in distant galaxies at different times in
the past have indicated that the → star formation rate
peaked at the → redshift of z ~ 2, some 10 billion years
ago, before declining by a factor of around ten to its
present value (P. Madau & Dickinson, 2014, arXiv:1403.0007).

A hypothetical stellar object whose structure or evolution has been affected by
→ dark matter. Dark matter models predict that in the
→ early Universe dark matter (in the form of
→ WIMPs) should congregate and annihilate in the cores
of → Population III stars. The result would be dark stars
with properties very different from ordinary stars. The reason is that the
presence of large amounts of dark matter during the formation of a
star inhibits the collapse and can partially prevent further
cooling of the gas beyond a certain critical point, well before a
→ main sequence object has formed. As a consequence,
dark matter stars should be more massive, more luminous, and live longer than
Pop. III stars, but would be cooler. The formation of dark stars could have had an
impact on the → reionization history of the Universe
(see, e.g., P. Scott, 2011, astro-ph/1101.1029, and references therein).

A star that lies within the → galactic disk of a
→ spiral galaxy.
Stars belonging to the → thin disk,
such as the Sun or Alpha Centauri, lie at a
typical distance of about 1,000 → light-years from the galactic
midplane. There are also → thick disk
stars, such as Lalande 21185, that lie at an
average distance of about 3,500 light-years from the midplane.

The hottest variety of stars on the → horizontal branch
with temperatures ranging from 20,000 to 40,000 K.
EHB stars are distinguished from normal horizontal branch stars by having extremely thin,
inert hydrogen envelopes surrounding the helium-burning core. They are
hot, dense stars with masses in a narrow range near
0.5 Msun. These stars have undergone such extreme mass loss during their first ascent up the
giant branch that only a very thin hydrogen envelope survives. Stars
identified as EHB stars are found in low metallicity globular clusters
as an extension of the normal HB.

A star with an iron abundance [Fe/H] < -3 found in a
→ galactic halo. These stars,
whose → metallicity is typically less than one thousandth
of the solar value, are believed to have formed shortly after the
→ Big Bang, 13.7 billion years ago. The number of such stars
depends on the primordial → initial mass function.
If the IMF were steep, there could, in principle, be a lot of EMPSs formed at high
→ redshifts. Thus many of them could have ended up in the
halos of galaxies. See also → Population III star.